1. Field of the Invention
The present invention relates to an electronically controlled differential limiting system, and specifically to a differential limiting system employing a clutch device for preventing wheel spin and loss of traction, even if one wheel encounters a patch of ice or snow and loses traction.
2. Description of the Background Disclosure
Recently, there have been proposed and developed various electronically controlled differential limiting systems for automotive vehicles, which act in an active manner for preventing wheel spin and loss of traction. Such electronically controlled differential limiting systems have been disclosed in Japanese Patent First Publications Tokkai (Showa) 61-59044 and 63-83435. The Japanese Patent First Publication Tokkai (Showa) 61-59044 will be hereinafter referred to as a "first prior art", while the Japanese Patent First Publication Tokkai (Showa) 63-83435 will be hereinafter referred to as a "second prior art". As is generally known, such conventional electronically controlled differential limiting systems include a differential case rotatably supported by bearings in a differential housing, a pair of differential side gears respectively splined to inner ends of axles, at least one differential pinion rotating on its pinion shaft whose ends lie in notches of the differential case, at least one set of clutch device provided between the side gear and the differential case so as to feed additional torque from the differential case through the clutch device to the slower-rotating differential side gear and to return torque from the faster-rotating differential side gear through the clutch device to the differential case, a fluid pressure operated clutch actuating device for engaging and releasing the clutch via fluid pressure. A multiple disc clutch is generally used as the clutch device. The multiple disc clutch has a first series of clutch plates mounted on the differential case and a second series of clutch plates mounted on the side gear. The first and second series of clutch plates are positioned alternately to form a multiple disc clutch. Such conventional differential limiting systems have some problems. With the traditional arrangement of the prior art differential limiting system, when the clutch device is engaged in the differential case, there is a tendency for considerably great pressures to be applied through clutch plates to the differential side gears. The pressure applied to the side gear results in an excessive engagement between the side gear and the meshed differential pinion. That is, such an excessive engagement tends to prevent normal tooth contact between the side gear and the pinion gear. There is another problem that the clutch device tends to receive reaction force transmitted from the differential pinion to the differential side gear when the pinion rotates on its shaft while meshing with the side gear. Such reaction prevents a precise differential limiting control. To avoid the former problem of an undesirable pressure applied to the side gear during operation of the clutch device, the previously noted first prior art differential limiting system includes a pair of symmetrically arranged pressure rings abutting to each other in an axial direction of the wheel axle. The pressure rings are disposed in a limited space defined in the differential case in such a manner as to operably enclose a pair of opposing differential side gears and a differential pinion meshing with the two side gears. The respective pressure ring has both ends facing apart from each other, one end abutting the adjacent pressure ring and the other end mating the innermost clutch plate of the clutch device. In the first prior art differential limiting system, when the left and right clutch devices are engaged, more of great engaging force of the left clutch is not transmitted to the left side gear but converted to pressure applied on the abutting end of the right pressure ring, while more of great engaging force of the right clutch is not transmitted to the right side gear but converted to pressure applied on the abutting end of the left pressure ring. These engaging forces required for application of the respective clutch device can be effectively received on the abutting surface between the adjacent two pressure rings. As set forth above, the first prior art differential limiting system provides an improved construction according to which the differential side gear is unaffected by the engaging force caused by the engagement of the clutch device. However, the arrangement of the first prior art is unsufficient since the the first prior art fails to provide means for solving the previously noted latter problem of a reaction transmitted from the differential pinion to the differential side gear during rotational movement of the pinion. To avoid the latter problem, the second prior art differential limiting system has a complicated configuration of a differential case in which the case is formed with a pair of substantially annular projections extending inwardly thereof in a radial direction of the side gear and offsetting to each other by a required distance, so as to rotatably receive each circumferential non-toothed, stepped section of the opposing side gears. In order to provide toothed sections of the opposing differential side gears between the two annular projections spaced apart from each other, the differential case is divided into three pieces firmly secured to each other by means of bolts. Such assembling is troublesome. Furthermore, the construction of the second prior art requires a relatively large number of parts of the differential, with the result that the entire weight of the differential is increased.
In the previously described electronically controlled differential limiting systems, the clutch actuating device is mounted on the differential housing. The clutch actuating device includes a pressure bearing for transmitting pressure necessary to engage the clutch operably disposed in the differential case, while absorbing the rotational speed difference between the housing and the outermost clutch plate receiving the pressure transmitted through the pressure bearing. The traditional pressure bearings are two types, namely a thrust bearing and a radial bearing. It is advantageous that the thrust bearing is utilized as a pressure bearing, since even a relatively small size of thrust bearing can reliably transmit a considerably great clutch-engaging force acting in a thrust direction of the clutch or in an axial direction of the wheel axle. However, in the differential limiting system employing a thrust bearing functioning as a pressure bearing, it is necessary that the thrust bearing is preloaded by means of a spring to prevent slippage occurring between the rolling element and the bearing ring. For example, if the preloading spring is provided between the thrust bearing and the clutch plates in series to the clutch plates, the clutch plates are also preloaded by the spring, with the result that the clutch plates generate an initial friction torque. The initial torque results in deterioration in a differential limiting control precision. With an undesirable arrangement of the preloading spring, there is a tendency for a spring-loaded surface to become damaged due to the rotational speed difference between the end of the spring and the spring-loaded member.
On the other hand, the differential limiting system employing a radial bearing functioning as a pressure bearing, does not require the previously mentioned preloading spring but require a large bearing size to reliably and sufficiently transmit a considerably great clutch-engaging force acting in the thrust direction.